dextercioby did it through definition of a derivative. if you are lazy like me, you can do it a shorter way!
[tex]y=x^3~~n=3[/tex]
if you have a polynomial function, and you want to find the derivative of it use the fact that if [itex]y=x^n[/itex], then [itex]\frac{dy}{dx} = nx^{n-1}[/itex].
[tex]\frac{dy}{dx}=3x^{3-1}=3x^2[/tex]
now, evaluate [itex]3x^2[/itex] at -2.
[tex]\frac{dy}{dx}=3(-2)^2=3(4)=12[/tex]
that's the lazy way.